1. Field of the Invention
The present invention relates to thermally stable catalysts based on metal oxides with a high dispersion of the active phase and with an improved specific surface, to their method of manufacture and to their applications in processes for the catalytic treatment of hydrocarbon and other compounds. It more particularly relates to the catalysts formed from supports containing zirconia.
2. Description of the Prior Art
A very large number of catalytic processes require the use of a catalyst comprising supports with a large specific surface, particularly sought for in oxidation processes and hydrotreating processes, on which are deposited a maximum of an active phase. The catalysts commonly used on an industrial scale generally consist of supports comprising one or a number of refractory oxides of metals such as zirconium, aluminium, silicon and titanium, the active phase consisting of transition metal sulphides or oxides. These catalysts are always prepared in at least two stages, a first stage of formation of the support and a second stage of impregnation of the latter with the active phase, the subsequent calcination and sulphurization treatments being targeted at converting the metal deposited in the salt form to oxides and to sulphides. The specific surface and the porosity of the support will depend on its method of manufacture. The dispersion of the active phase on this support will be better or worse according to whether the specific surface and the porosity of the support are larger or smaller and according to the experimental conditions of impregnation of the support. In particular, the interaction of the metal salt with the support is directly related to the acid/base properties of the support. Thus, when all the accessible sites of the support are occupied by the active metal ion of the salt present in the impregnation solution, the metal ion remaining in solution can no longer be deposited on the support in an evenly dispersed active form. In these conventional-type catalysts, the amount of active phase is therefore very quickly limited by the quality of the support, that is to say its capacity to absorb active metal ions, expressed by the number of metal atoms deposited per unit of surface area, which has been confirmed by many writers and experimentally revealed by analytical means such as UV spectroscopy and by X-ray Photoelectron Spectroscopy (XPS).
As the efficiency of a catalyst depends not only on the conditions of implementation of the catalytic process but also on its activity, which is directly related to the amount of active phase present on the support, many preparation methods have been proposed either for increasing the surface area of the support or for modifying its surface condition.
For certain supports, especially those containing zirconia, it has not been possible to obtain a specific surface as high as that of the most commonly used supports containing alumina. Moreover, supports based on zirconia have proved to have little thermal and hydrothermal stability. In the presence of structural dopants such as yttrium, lanthanum or silica, this thermal stability has been improved but still insufficiently.
In order to solve this problem of thermal instability of zirconia and in order to improve the specific surface of the catalysts containing zirconia, there has been developed a specific synthesis of supports containing refractory metal oxides, described in French Patent Application 2,661,169 of 20 Apr. 1990. It consists in heating a metal salt in a molten salt medium consisting of at least one alkali metal salt with an oxidizing effect at a temperature greater than that of the eutectic. By this process, it was possible to synthesize supports based on oxides of metals from Groups III and IV of the Periodic Table and, more particularly, supports based on refractory oxides such as zirconium, aluminium, silicon and titanium oxides, these oxides precipitating in the molten salt medium.
In order to further improve the specific surface of these supports, the idea was developed of preparing supports composed of mixed oxides and it was observed that the metal atoms distributed themselves homogeneously and evenly in the support for a not insignificant improvement in its specific surface. Of course, a catalytically active metal phase was subsequently deposited on these supports in a conventional way.
Contining this work on catalysts whose support is obtained by the so-called molten salts method, led to testing of a number of possible combinations of metal oxides with zirconia and it was noticed that certain combinations, especially combinations with salts of catalytically active metals, make it possible to increase, in an entirely unexpected way, the specific surface of zirconia-based supports. Moreover, it was discovered that these supports, which have become active by their composition and which are thermally stable, can be used directly as catalysts.
The present invention is therefore targeted at thermally stable catalysts for oxidizing and hydro-treating hydrocarbons comprising zirconium oxides with a higher specific surface than that known by those skilled in the art for a better dispersion of the active sites. The specific surface of these catalysts is measured by the BET method corresponding to the ASTM standard D3663.